The hardness and tensile properties of Mg-La-Zr alloys with various lanthanum contents were investigated, and the microstructures of the alloys were examined. The microstructure was composed of fine globular primary α-Mg grains and eutectic areas. The values of yield stress and modulus of the eutectic were, respectively, about 3 times and 1.7 times higher than those of the primary α-Mg. The increase in the yield stress of the alloys with less than 1.2%La is due to a rapid increase in coverage of the α-Mg grain boundary by the eutectic. As the increment in the grain boundary coverage decreases with lanthanum content, the composite strengthening (composite materials effect) starts to play a more significant role in increasing yield stress. As the grain boundary coverage reaches a plateau at about 2%La, further increase in the yield stress is mainly due to the composite strengthening, the effect of which is dependent on the volume fraction of eutectic. During tensile-testing at 150℃, the alloys with higher lanthanum content exhibited dual yield points, the first from yielding of the primary α-Mg, followed by the higher-yield point associated with the eutectic. This suggests that composite strengthening is in effect.

This Paper was Originally Published in Japanese in J. JILM 66 (2016) 647-651.